Packaging for microleds for chip to chip communication

ABSTRACT

A microLED based optical chip-to-chip interconnect may optically couple chips in a variety of ways. The microLEDs may be positioned within a waveguide, and the interconnects may be arranged as direct connections, in bus topologies, or as repeaters.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 62/958,619, filed on Jan. 8, 2020,and 62/960,546, filed on Jan. 13, 2020, the disclosures of which areincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to chip-to-chip opticalinterconnects, and more generally to chip-to-chip optical interconnectsincluding microLEDs.

BACKGROUND OF THE INVENTION

Logic and memory capabilities may be limited by electricalinterconnects. As the requirement for dataflow increases, especially inmachine learning and other high-performance applications, chips may notbe packaged individually, but combined into multi-chip modules. Insidethese multi-chip modules, which in some cases may be considered SiPs(System in Package), multiple chips are connected using very densewiring. For example, logic and memory may be mounted on a siliconinterposer using microbumps. However, even such interposers may beinsufficient to connect the desired number of ICs together at highenough data rates. The connections within these packages may be limitedby the parasitic resistance, inductance, and capacitance of the wiringand the chips may need to be mounted very close to each other to reducethose effects. This could limit the number of ICs that can be combinedin a multi-chip module. In addition, wiring density also posesconstraints. If the number of lanes for data communication is reduced byincreasing the speed per lane, then the parasitics become worse andadditional power is generally consumed in the SERDES to multiplex datato higher rates.

BRIEF SUMMARY OF THE INVENTION

Some embodiments provide optical links between integrated circuits (ICs)using light from microLEDs. In some embodiments the microLEDs arepackaged so as to improve light transmission characteristics.

Some embodiments provide a chip-to-chip optical interconnect including amicroLED, comprising: a first semiconductor chip electrically coupled toan interposer; a second semiconductor chip electrically coupled to theinterposer; the interposer including electrical signal pathselectrically coupling the first semiconductor chip and the secondsemiconductor chip; a first microLED; first circuitry, electricallycoupled to the first semiconductor chip, for driving the first microLEDbased on data from the first semiconductor chip; a first encapsulantsubstantially encapsulating the first microLED; a first photodetector;first amplification circuitry for amplifying signals from the firstphotodetector, the first amplification circuitry electrically coupled toprovide electrical signals to the second semiconductor chip; a firstwaveguide optically coupling the first microLED and the firstphotodetector, the encapsulated first microLED within material of thefirst waveguide.

Some embodiments further provide a second microLED; second circuitry,electrically coupled to the second semiconductor chip, for driving thefirst microLED based on data from the second semiconductor chip; asecond encapsulant substantially encapsulating the second microLED; asecond photodetector; second amplification circuitry for amplifyingsignals from the second photodetector, the second amplificationcircuitry electrically coupled to provide electrical signals to thefirst semiconductor chip; the first waveguide optically coupling thesecond microLED and the second photodetector, the encapsulated secondmicroLED within material of the first waveguide.

Some embodiments instead or in addition further provide a secondphotodetector; and second amplification circuitry for amplifying signalsfrom the second photodetector, the second amplification circuitryelectrically coupled to provide electrical signals to a thirdsemiconductor chip; and wherein the first waveguide optically couplesthe first microLED and the second photodetector.

Some embodiments instead or in addition further provide a secondmicroLED; second circuitry, electrically coupled to the firstamplification circuitry, for driving the second microLED based on datafrom the first photodetector; and a second optical waveguide opticallycoupling the second microLED and a third photodetector.

These and other aspects of the invention are more fully comprehendedupon review of this disclosure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an example of two chips in data communication, inaccordance with aspects of the invention.

FIG. 2 shows a further example of two chips in data communication, inaccordance with aspects of the invention.

FIG. 3 shows an example basic architecture of using microLEDs for chipto chip interconnects in a 2D format, in accordance with aspects of theinvention.

FIG. 4A illustrates a top view of a structure using encapsulation andreflectors for increasing coupling of light from a microLED into amultimode waveguide, in accordance with aspects of the invention.

FIG. 4B illustrates a side cross-sectional view of the structure of FIG.4A.

FIG. 5A shows a microLED pillar mounted on a silicon substrate withmetallization above, below, and to the sides, in accordance with aspectsof the invention.

FIG. 5B is a 3D illustration of a microLED pillar showing a rearreflector wrapped around the sides, deflecting the light in a forwarddirection, in accordance with aspects of the invention.

FIG. 6 illustrates a side view of a structure using encapsulation andreflectors with a spherical reflective geometry in a vertical directionfor increasing coupling of light from a microLED into a multimodewaveguide, in accordance with aspects of the invention.

FIG. 7A shows a perspective view of a half-duplex microLED basedtransmitter and receiver coupled to a solder ball, in accordance withaspects of the invention.

FIG. 7B is an expansion of portions of the device of FIG. 7A, with thewaveguide not shown for clarity.

FIG. 8 illustrates a multiplicity of the optical interconnects of FIG.7A, in accordance with aspects of the invention.

FIG. 9 shows a microLED-based TX coupled to a waveguide, with a splitterto multiple RXs, in accordance with aspects of the invention.

FIG. 10 shows an implementation of three nodes (each with a TX and anRX) all connected through a star topology, in accordance with aspects ofthe invention.

FIG. 11 shows a side view of an example microLED-basedoptical-to-electrical-to-optical (OEO) repeater, in accordance withaspects of the invention.

FIG. 12 shows an example top view of a bidirectional repeater-basedinterconnect, in accordance with aspects of the invention.

FIG. 13 shows an example of internal logic of a repeater, in accordancewith aspects of the invention.

FIG. 14 shows a semi-schematic of an example implementation of an arrayof drivers and receivers between an FPGA and an HBM stack, in accordancewith aspects of the invention.

FIG. 15 illustrates an example of integrating an FET driver with amicroLED, in accordance with aspects of the invention.

FIG. 16 shows an example of a detector integrated with an amplifier, inaccordance with aspects of the invention.

DETAILED DESCRIPTION

Some embodiments provide optical links between the integrated circuits(ICs, and microLEDs are particularly useful for this application. Byusing very dense arrays of optical links running at slower speed, theremay be reduced or no need for serializer-deserializers (SERDES) and highdata rates can be obtained with lower power consumption. There are alsogenerally no parasitic circuit limitations or electrical crosstalk inoptical links. Such optical interconnects can be realized in 2D formatin waveguides or in 3D using lens arrays, holograms, or 3D waveguides.The light from microLEDs is quite different from that of lasers, inemission pattern, spectrum, and other features, and thus differentpackaging architectures may be preferred for use of microLEDs for chipto chip communications.

Optically-Enhanced Interposers

FIG. 1 shows two chips 111, 113 in data communication. In someembodiments the chips are microchips. In some embodiments the chips arepart of a same multi-chip module. In some embodiments the chips are in asame package. In some embodiments the chips are on a same substrate, orcoupled to the same substrate. The two chips are interconnectedtogether, but instead of conventional electrical links, there arenumerous transmitter (Tx) 115 and receiver (Rx) 117 blocks that convertthe electrical signals to optical signals and back again. The links 119between the Tx blocks and the microchips are electrical, as are thelinks 121 between the Rx blocks and the microchips. But the connections123 between the Tx blocks and Rx blocks are optical. The Tx blocksinclude microLEDs for generating the optical signals based on electricalsignals, and the Rx blocks include photodetectors for generatingelectrical signals based on received optical signals. FIG. 1 showsindividual Tx and Rx blocks, but multiple Tx and Rx blocks can beintegrated together on a single integrated circuit.

Frequently microchips use the same electrical pin for both input andoutput, generally in half-duplex more, where either the electrical lineis transmitting or it is receiving. Various handshaking routines may beused to transmit and receive using the same lines. The same can beapplied in the optical domain as shown in FIG. 2. Here each electricalline is connected to a Tx block 115 and an Rx block 117. In someembodiments logic 211 connects the two together, such that when data isbeing sent the Tx block is activated and when it is received, the Rxblock is activated. In some embodiments a separate optical line orwaveguide is used for transmission and another optical line or waveguideis used for reception, as is shown in the figure. Alternatively, in someembodiments a single waveguide or optical connection is used to carrydata in both directions. The latter option may provide higher density,as fewer waveguides may be used. However, there may be some loss or“blocking” associated with using the same line for transmit and receive.

FIG. 3 shows the basic architecture of using microLEDs for chip to chipinterconnects in a 2D format. The various chips 311, 313 to beinterconnected may be microbumped onto an optical interposer assemblythat is comprised of a driver 363, a microLED 361, an opticalpropagation medium for example in the form of a waveguide 323,photodetector 371 and an amplifier 373. In some embodiments the chipsmay be silicon processors. In some embodiments the chips may include aCPU, a GPU, and/or memory. The chips 311, 313 are mounted to theinterposer, for example using solder balls and, in some embodiments,additional items. In FIG. 3, the chip 311 may provide data to the driver363. The driver activates the microLED 361 so as to generate lightencoding the data, with the light entering a first end of the waveguide323 and passing through the waveguide to a second end of the waveguide.The photodetector 371 is at the second end of the waveguide andgenerates electrical signals based on the received light. The amplifieramplifies the electrical signals from the photodetector, with the dataof the amplified signals provided to the second chip. Though thecomplexity is more than that of just a “wire”, the link can generallyoperate at lower powers since the capacitance at both the source anddestination chip ends is generally greatly reduced relative to anelectrical connection. The interposer 351 may still have basicelectrical lines such as power for the drivers and the receiver, groundline, and other control signals that are relatively slow. Mixedimplementations are also possible, where some of the high-speed lines,for example some of the high speed lines between the chips 311, 313, areelectrical and some are optical. There may be vias through theinterposer substrate (e.g. through-chip-vias, TCVs orthrough-substrate-vias, TSVs) for many of the power and signal lines.There could be electrical connections from the back of the chip to theinterposer through solder bumps (e.g. controlled collapse chipconnection or “C4” bumps) and also from the top of the chip to theinterposer through wirebonds. Thus, the optics can enhance an electricalinterposer rather than completely replace all the electrical lines.

Optimizing microLEDs with Waveguides for Optical Interconnects

Unlike the emission from a laser that is typically confined to arelatively small number of spatial modes and is relatively directional,generally the emission from a microLED is Lambertian or omnidirectional.The light in a microLED is generated internally in the high indexmedium, typically a III-V semiconductor, and therefore can suffersignificant total internal reflection and be difficult to extract. Infact, with no additional modifications, the total internal reflectionconstraint in a high index material typically limits the extractionefficiency to only a few percent. For lighting applications, the LEDsurface may be roughened to reduce the total internal reflection loss.The LED may also be placed on a reflective surface to make use of lightsent to the rear of the device. In small devices, reflectors may beplaced to the sides to also direct light emitted laterally to the frontof the device. Since it is difficult to shape the LED into a sphericalgeometry, encapsulation of the LED into a spherical mold using a highindex medium such as polymer or epoxy can also be very useful inincreasing extraction efficiency as the total internal reflectioncriterion is reduced.

In the following discussion, a transmitter (TX) comprises a microLED anda microLED driver electrical circuit that drives the microLED. Areceiver (RX) comprises a photodetector followed by a receiverelectrical circuit, where a typical receiver circuit includes atransimpedance amplifier (TIA) followed by a limiting amplifier (LA).Such a TX and RX may also be considered as examples of the TX blocks andRX blocks of FIGS. 1 and 2.

A microLED is made from a p-n junction of a direct-bandgap semiconductormaterial. A microLED may be distinguished from a semiconductor laser(SL) in the following ways: (1) a microLED does not have an opticalresonator structure; (2) the optical output from a microLED is almostcompletely spontaneous emission whereas the output from a SL isdominantly stimulated emission; (3) the optical output from a microLEDis temporally and spatially incoherent whereas the output from a SL hassignificant temporal and spatial coherence; (4) a microLED is designedto be operated down to a zero minimum current, whereas a SL is designedto be operated above a minimum threshold current, which is typically atleast 1 mA.

A microLED may be distinguished from a standard LED by (1) having anemitting region of less than 100 μm×100 μm; (2) typically havingpositive and negative contacts on top and bottom surfaces, whereas astandard LED typically has both positive and negative contacts on asingle surface; (3) typically being used in large arrays for display andinterconnect applications.

FIGS. 4A and 4B illustrate structure(s) for increasing coupling of lightfrom a microLED 411 into a multimode waveguide 415. FIGS. 4A and 4B usean example of a polymer waveguide. In various embodiments, differentwaveguide materials can be used such as germanium-doped silicon dioxidewaveguides, silicon nitride/oxide waveguide, or other materials. Thecladding can be material of lower index, air, the oxidized surface ofthe silicon wafer, or even highly reflective mirrors such asanti-resonant reflective optical waveguides (ARROW). In someembodiments, top and side claddings may be of air, with a bottom polymercladding.

In some embodiments the microLED can be encapsulated in an encapsulant413, which may be a high index material, or formed in the waveguideitself. For example, a polymer waveguide can be formed in polymer withan opening. The microLED can be bonded into this hole. The hole can thenbe filled with an encapsulant, for example a silicone elastomermaterial. The encapsulant may increase the extraction efficiency byreducing the total internal reflection constraint.

In some embodiments reflectors are placed below and around the microLEDto direct the light into the waveguide. The bottom surface 451 a onwhich the microLED is bonded can be reflective, for example with a metalsuch as silver or aluminum. The top surface of the microLED can also becoated with a reflective contact 451 b. Without these reflectors, lightthat would be emitted vertically up or down from the microLED generallymay be lost. These reflectors send the light back into the structurewhere the photons could be scattered into the waveguide, or if absorbedto generate electron-hole pairs that could in turn be re-emitted intothe desired optical mode. In either case, reflectors can help to steerlight emitted in undesired directions into directions that can propagatedown the waveguide and thus increase the efficiency of the coupling intothe waveguide. A rear edge of the waveguide, away from a photodetector419 about an opposing end of the waveguide, may also include ametallized mirror 417, for example a mirror with a parabolic shape. Thephotodetector may have electrical connections 421 a,b.

Metallization around the microLED itself can be very effective. FIG. 5Ashows a microLED pillar 511 (indicated in FIG. 5B) mounted on a siliconsubstrate 561 with metallization above, below, and to the sides. ThemicroLED includes an N-type GaN layer 553 on an n-metal contact 519, anda P-type GaN layer 551 under a p-metal contact 513. The P-type GaN layerand the N-type GaN layer sandwich an intrinsic GaN region 555 with InGaNquantum wells. The p-metal contact includes a portion 515 that extendsdown the microLED, to provide a portion 517 on a silicon dioxide layer559 on which the n-metal contact also rests. The silicon dioxide layeris on a top of the silicon substrate. A passivating layer 557 around themicroLED can prevent shorting between the p-metal and the n-metal. Awindow on the front allows the light to escape in the right direction(as viewed in FIG. 5). FIG. 5B is a 3D illustration of a device such asthe device of FIG. 5A showing how the rear reflector 515 (e.g. thep-metal) can wrap around the sides, deflecting the light in the forwarddirection. Selective metallization on the rear of the microLED can beimplemented using angled evaporation or other lithographic technique.Passivation could be AlN, SiN, SiO2, or other dielectric materials, andcould be all around the pillar as well as to the rear as typical filmsare transparent at the wavelength of interest. An appropriate thickness(¼ wave) could also be used as an anti-reflection coating furtherenhancing optical extraction efficiency.

In some embodiments the back of the waveguide is coated with areflective layer. Ideally, the back surface can form an approximatelyparabolic mirror, with the microLED at the focal point and a reflectivecoating 417 placed to the rear of the waveguide as shown in FIG. 4A.These reflectors redirect the light going backwards into the desiredforward direction.

The photodetector on the other end of the waveguide can be butt-coupledto the waveguide with the detecting area covering the waveguidecross-section. Alternatively it could be placed under the waveguide oron the sides, as all the beams impinge on the sides as well as the rearof the waveguide. This is further explained in subsequent figures.

Note that the curvature and reflective geometry in FIG. 4A can also beapplied in the vertical direction with appropriate shaping of thewaveguide. In some embodiments an approximately parabolic reflector isformed in the waveguide top and even bottom surface as well as thesides. The encapsulant can also be shaped optimally for maximumextraction efficiency. Ideally, the microLED is spherical in shape, butbarring that, encapsulants or properly shaped reflector(s) andwaveguides can optimize coupling into waveguide modes. FIG. 6 shows howa spherical reflective geometry could be used in a vertical direction.In FIG. 6, a microLED 611 is encapsulated by an encapsulant 613. ThemicroLED and encapsulant are within and near a first end of a waveguide615, which may be a polymer waveguide. The first end of the waveguidehas a spherical geometry in at least a vertical direction (with a lengthof the waveguide from the first end to the second end being in ahorizontal direction), with a metallized mirror 617 formed on the firstend of the waveguide. A photodetector 619 is at a second end of thewaveguide, with the photodetector butt-coupled to the second end of thewaveguide in FIG. 6. The waveguide is on an SiO2 layer 659, on top of asilicon substrate 661. The SiO2 layer acts as a lower cladding for thewaveguide. Metal trace 652 on the SiO2 layer electrically connects themicroLED to a microLED driver 612, also on the SiO2 layer. Similarly,metal trace 621 connects the photodetector to a transimpedance amplifier622, also on the SiO2 layer.

FIG. 7A shows a 3D view of a half-duplex microLED based transmitter andreceiver connected to a solder ball. In FIG. 7A, an optical waveguide715 extends over a portion of an RX/TX chip 771 and an SiO2 layer 759 ona silicon wafer 761 on which the RX/TX chip sits. The RX/TX chip iselectrically connected to the solder ball by a first electrical line746, with the RX/TX chip also connected to a positive voltage line 742and a ground line 744, all of which are on the SiO2 layer. In someaspects, embodiments in accordance with FIG. 7A may be considered asreplacing electrical wires between solder balls (or other types ofelectrical connectors) in a multi-chip module with an optical waveguideand very simple driver and receiver components. The large substrate inthe figure could be a silicon or glass wafer coated with a thick layerof silicon dioxide. In some embodiments the silicon dioxide may act bothas a lower cladding for the waveguide and also as an electricalinsulator. The solder ball, which could connect to a similar ball on alogic or memory IC, is electrically connected to the RX/TX microchipthat has the RX and TX functions. This microchip could be transferredonto the substrate, or could even be made in the substrate itself. Oneor more lithographic steps connects this microchip to power line,ground, and a signal line to the solder ball. There could be otherelectrical connections to the chip, such as clock or signals forequalization or enhancement of the transmitter or receiver signals. Anoptical waveguide is formed on the substrate and is coupled to themicrochip.

FIG. 7B is an expansion of portions of the device of FIG. 7A, with thewaveguide not shown for clarity. The microchip includes 4 functionalelements. There is a microLED 711 mounted on the microchip, for exampleas described previously with respect to FIGS. 4A and 4B, with rear andtop metallization to emit the light in the forward (down the waveguide)direction. This microLED is electrically connected to a driver circuit712 on the microchip. This driver can be a very simple transistor thatchanges the signal level from the source to an appropriate voltage orcurrent to drive the microLED. It could also have other functions, suchas a way to set the bias and modulation voltage, enhancement of higherfrequencies, or the ability to drive a reverse bias to sweep outcarriers to enhance speed. Also on the microchip is a photodetector 719to receive the optical signal in the waveguide. There are variousgeometries of photodetectors, such vertical p-i-n detectors, metalSchottky diode detectors or lateral metal-semiconductor-metal detectors.In this case, FIG. 7 shows interdigitated fingers of n+ and p− regionsformed in lower doped semiconductor to form a lateral p-i-n regions. Thetwo electrical connections from the detector are biased and thephotocurrent measured by a transimpedance amplifier 722 formed in themicrochip. Since the detector is integrated with the transimpedanceamplifier, there is very low capacitance in the connection between thetwo and low power and high speed performance can be obtained. Alsopresent on the microchip is some logic, which in some embodiments couldbe as simple as a diode that decides whether the microchip acts as atransmitter, activating the microLED or as a receiver, taking signalsfrom the photodetector.

In the embodiment of FIG. 7A, the waveguide is significantly larger thanthe microLED and the entire microLED is contained within the waveguide.As a transmitter, the microLED sends data down the waveguide. Behind themicroLED is the detector on the bottom side. As a receiver, most of thelight passes by the microLED, reflecting on the different surfaces ofthe waveguide, and is finally absorbed in the lower layer between theinterdigitated fingers and generates photocurrent. There is some loss aspart of the incoming light is blocked by the microLED, but if enoughlight is received to achieve a desired signal to noise ratio, anadequate bit-error-rate can be obtained.

There are numerous variations on this. For example, some embodimentshave two separate waveguides, one for transmission and one for receivingsignals. The microLED would be connected to one waveguide and thedetector would be connected to the other waveguide.

The microLED could itself be used a detector with a reverse bias appliedto the diode to generate photocurrent, or other photodetectorsstructures could be implemented.

FIG. 8 illustrates a multiplicity of the optical interconnects of FIG.7A. A logic or memory chip 811 would typically have multiple rows ofmicrobumps or solder balls 813 underneath that connect to the siliconinterposer. In this case each of the signals is routed to a Tx/Rx chip815 and an optical waveguide 817. The waveguides from all of these bumpstransport the signals from one area to another.

Diversity microLED-Based Optical Interconnects

Once the light is in the waveguide, many optical functions may beperformed optically rather than electrically, and this may have numerousadvantages. For example, the function of a splitter is often difficultto do in the electrical domain, as impedance discontinuities, splittingratios, time delays, and cross-talk to other lines can be problematic. AmicroLED can be coupled to a waveguide followed by an optical splitterused to connect to multiple detectors. These splitters can use starcouplers or other structures. FIG. 9 shows a microLED-based TX coupledto a waveguide, with a splitter to multiple RXs. In FIG. 9 an electricalconnection 913 is coupled to a microLED driver 912. The microLED driverdrives a microLED 911 in a waveguide 915. A fraction of the opticalpower can be split off the main waveguide by an optical tap and sent toan Rx. Multiple taps/RXs can be cascaded to implement a tapped bus. InFIG. 9, a plurality of taps are shown, with each tap terminating in aphotodetector 919 a-d, coupled to a transimpedance amplifier 922 a-d, inturn coupled to an electrical connection 924 a-d. Alternatively, asingle 1-to-N optical splitter can be used to connect one TX to multipleRXs. In some embodiments the multiple RXs are for multiple logicalunits, for example as may be found in AI/Machine learning/neuralnetworking applications. In some embodiments the multiple RXs are formultiple multiplication units, for example used for matrixmultiplication, in which each number may be multiplied by N othernumbers and the results summed. By having the splitter in the opticaldomain, the same signal can be accurately sent to multiple receivers.This can also work in the analog domain, where optical signals aredivided, subtracted, or in reverse, two signals can be added. Opticalsplitters can be very accurate and split into tens, hundreds, orthousands of waveguides, maintaining signal integrity through thesplits.

Though FIG. 9 shows microLED at the end of one waveguide and detectorsin other positions to implement simplex 1-to-N connectivity, the linksin FIG. 9 can be operated in duplex mode (either full or half) byplacing both a microLED and a detector at the end of each waveguide.Since the waveguides are generally multimode, part of the waveguide canbe connected to a source and another to the receiver. For memory accessand some other applications, the waveguides may be operated inhalf-duplex where only one source is transmitting at a given time.

FIG. 10 shows an implementation of three nodes (each with a TX and anRX) 1011 a-c all connected through waveguides 1015 in a star topology,so any node can transmit to the others and any can receive from anyother. Each node is coupled to a microbump 1013, for passage ofelectrical signals to and from a chip. Each node includes a microLED1011 and photodetector 1019, with a driver 1012 for driving the microLEDbased on signals received via the microbump and a transimpedanceamplifier 1022 for amplifying signals from the photodetector. Whenoperated in half-duplex mode, each microbump provides an inputelectrical signal to the TX or gets an electrical output signal from theRX, depending on whether that node is transmitting or receiving. The TIAand driver may be powered using voltages available on the interposer. Inthis architecture, there is some loss associated with the splitters, buta sufficiently powerful microLED can provide enough optical power toovercome the extra losses in the link and achieve a useful receivedsignal-to-noise ratio (SNR) and/or bit-error ratio (BER). While FIG. 10shows only three nodes, the architecture can be extended to largernumbers of nodes, constrained primarily by the ability to obtain anadequate SNR/BER at each RX.

Repeaters in microLED-Based Optical Interconnects

When the splitters become large, or the number of connections become toonumerous, one or more repeaters of some kind may be used to maintainadequate signal amplitude and SNR. One application is the connectionbetween logic to multiple memory modules. In a typical computerarchitecture, the processor data lines are shared among multipleperipherals and memory, and a chip-select line is used to turn on thevarious modules. This may not be preferable with closely packed memorysuch as High-Bandwidth Memory (HBM) because the electrical link arelimited to short lengths (often <10 mm) and electrically splitting onesignal to multiple modules may cause too much signal impairment.

Some embodiments provide an optical approach, with a microLED basedoptical-to-electrical-to-optical (OEO) repeater, for example generallyillustrated in FIG. 11. In FIG. 11, a logic chip 111 provides a signalby way of a solder ball 1153 to an RX/TX chip to drive an optical source1161 a. The optical source may be a microLED. The optical source iscoupled to a waveguide 1115 a and terminates at photodetector 1119 a ata first module 1113 a, an HBM stack in FIG. 11. The signal may beprovided to the HBM stack by way of a solder ball 1153 b. At that sametermination point, the signal is regenerated by an RX/TX chip 1150 a toanother microLED 1161 b that couples to a second waveguide 1115 b andreaches a photodetector 1119 b coupled to an RX/TX chip 1150 b at asecond HBM stack 1113. The signal may be provided to the second HBMstack by way of a solder ball 1153 c. In this way multiple memory orother types of modules can be cascaded. In FIG. 11, the RX/TX chip, themicroLEDs, the waveguides, and the photodetectors are shown on an SiO2layer on top of a silicon substrate.

FIG. 12 shows an example top view of a bidirectional repeater-basedinterconnect, for example that of FIG. 11. In FIG. 12, a logic chip1211, a first HBM stack 1213 a, and a second HBM stack 1213 b each haverepeaters, with the repeaters for the logic chip and first HBM stackcoupled by waveguides 1215 a,b and the repeaters for the first HBM stackand the second HBM stack coupled by waveguides 1215 c,d. For example, amicroLED 1261 a is provided at the logic chip end of the waveguide 1215b and a photodetector 1219 a is provided at the first HBM stack end ofthe waveguide 1215 b, while a microLED 1261 b is provided at the firstHBM stack end of the waveguide 1215 a and a photodetector 1219 b isprovided at the logic chip end of the waveguide 1215 a. Thebidirectional interconnect uses two repeaters, where each repeater hasan input waveguide optically coupled to an RX, which iselectrically-connected to a microLED-based TX, which is opticallycoupled to an output waveguide. FIG. 12 shows the use of separatewaveguides for each direction of the bidirectional link. However, theconnection can also be implemented with just one waveguide used for bothdirections of the link.

FIG. 13 shows an example of internal logic of the repeater. When data ismoving from left to right, the left most receiver 1319 c and TIA isactive and the left most light emitter 1317 a (microLED) and driver areoff, and the opposite on the right most receiver 1319 b and right mostemitter 1317 b. When the light is moving from right to left, the logicis reversed, as shown in the table. If there is some additional logic1313 incorporated into the receiver, one could simply repeat the signalfrom either direction, ignore and terminate the incoming signal andgenerate a new signal, or simply block the signal. This becomesequivalent to a more powerful version of “tri-state” logic where a chipcan get control of a bus, disengage from the bus, or even just terminatethe bus.

Electronic Integration

In some embodiments drivers and receivers are monolithically integratedinto an array on an IC. FIG. 14 shows a semi-schematic of an arrayimplementation between an FPGA 1411 and an HBM stack 1419 where a singleIC is used on either end. Signals to and from the FPGA are provided on abus to a first TX/RX ASIC 1413, which drive LEDs and process signalsfrom photodetectors. Similarly, signals to and from the HBM stack areprovided on a bus to a second TX/RX ASIC 1417, which drive LEDs andprocess signals from photodetectors. A plurality of waveguides 1415couple corresponding LEDs and photodetectors associated with the ASICs1413, 1417. An advantage of an array on an IC is that timing signals canbe shared along with other control signals, such as those used for bias,clock, pre-emphasis and equalization.

The driver could also be monolithically integrated with the microLEDitself. As FIG. 15 illustrates a combination device of a microLEDtogether with an enhanced-mode FET. The combination device is shownschematically on the left and a cross section on the right. A smallvoltage on the gate creates a connection in the accumulation region andconnects the source to the drain. This drives current through themicroLED and turns the microLED on.

There are numerous ways of integrating the FET with the microLED. Oneimplementation is shown in FIG. 15. The microLED includes an n-GaN layer1512 and a p-GaN layer 1510 sandwiching an intrinsic InGaN regionincluding quantum wells 1514. In FIG. 15 the LED is flipped upside down,with the p-GaN layer on a silicon substrate 1530, and a 2D accumulationregion is created in the p-type GaN by depositing AlGaN on the top. Someof this material is removed for a gate 1522. AlGaN has the effect ofcreating an n-type accumulation region below it, which are connected tothe source 1526 and the drain 1524. A positive voltage on the gate makesa connection between the two accumulation regions and thereby groundsthe cathode of the microLED, turning it on.

FIG. 16 shows a further example of a detector integrated with anamplifier. In FIG. 16 a silicon microchiplet 1619 is on a silicondioxide layer 1613 of a silicon substrate 1611. An optical waveguideextends over the silicon substrate and at least part of the siliconmicro-chiplet, a part that will serve as a photodetector. As illustratedin FIG. 16, a lightly doped silicon layer is formed into themicrochiplet 1619, with one part of the microchiplet serving as adetector 1617 and another part of the microchiplet acting as an FET 1621that can amplify the photocurrent. There are a host of possible circuitconfigurations that are possible to integrate monolithically.

Although the invention has been discussed with respect to variousembodiments, it should be recognized that the invention comprises thenovel and non-obvious claims supported by this disclosure.

What is claimed is:
 1. A chip-to-chip optical interconnect including amicroLED, comprising: a first semiconductor chip electrically coupled toan interposer; a second semiconductor chip electrically coupled to theinterposer; the interposer including electrical signal pathselectrically coupling the first semiconductor chip and the secondsemiconductor chip; a first microLED; first circuitry, electricallycoupled to the first semiconductor chip, for driving the first microLEDbased on data from the first semiconductor chip; a first encapsulantsubstantially encapsulating the first microLED; a first photodetector;first amplification circuitry for amplifying signals from the firstphotodetector, the first amplification circuitry electrically coupled toprovide electrical signals to the second semiconductor chip; a firstwaveguide optically coupling the first microLED and the firstphotodetector, the encapsulated first microLED within material of thefirst waveguide.
 2. The chip-to-chip optical interconnect of claim 1,wherein the first waveguide is on a surface of the interposer.
 3. Thechip-to-chip optical interconnect of claim 2, wherein the opticalwaveguide comprises a polymer waveguide.
 4. The chip-to-chip opticalinterconnect of claim 1, wherein the encapsulated first microLED is in ahole in the waveguide.
 5. The chip-to-chip optical interconnect of claim1, wherein the first microLED includes a rear surface facing away from apath in the optical waveguide towards the first photodetector, and thefirst microLED includes metallization on the rear surface so as toreflect light towards the first photodetector.
 6. The chip-to-chipoptical interconnect of claim 1, wherein the optical waveguide includesa back surface, the back surface optically in a direction opposite thefirst photodetector with respect a position of the first microLED, theback surface including a reflector to reflect light towards the firstphotodetector.
 7. The chip-to-chip optical interconnect of claim 1,further comprising: a second microLED; second circuitry, electricallycoupled to the second semiconductor chip, for driving the first microLEDbased on data from the second semiconductor chip; a second encapsulantsubstantially encapsulating the second microLED; a second photodetector;second amplification circuitry for amplifying signals from the secondphotodetector, the second amplification circuitry electrically coupledto provide electrical signals to the first semiconductor chip; the firstwaveguide optically coupling the second microLED and the secondphotodetector, the encapsulated second microLED within material of thefirst waveguide.
 8. The chip-to-chip optical interconnect of claim 7,wherein the first microLED is positioned in an optical path from thesecond microLED to the second photodetector and the second microLED ispositioned in an optical path from the first microLED to the firstphotodetector.
 9. The chip-to-chip optical interconnect of claim 1,wherein the first semiconductor chip includes a processor and the secondsemiconductor chip is a memory chip.
 10. The chip-to-chip opticalinterconnect of claim 1, further comprising: a second photodetector; andsecond amplification circuitry for amplifying signals from the secondphotodetector, the second amplification circuitry electrically coupledto provide electrical signals to a third semiconductor chip; and whereinthe first waveguide optically couples the first microLED and the secondphotodetector.
 11. The chip-to-chip optical interconnect of claim 1,further comprising: a second microLED; second circuitry, electricallycoupled to the first amplification circuitry, for driving the secondmicroLED based on data from the first photodetector; and a secondoptical waveguide optically coupling the second microLED and a thirdphotodetector.
 12. The chip-to-chip optical interconnect of claim 1,wherein the first circuitry is monolithically integrated with the firstmicroLED.